Untold Secrets Special Series...
Get ready for...
Here's a peek behind the scenes, inside
Chevrolet Engineering during a time when the stakes were high and winning was
The end result: Two
consecutive Trans-Am championships and one of the most potent small block power packages
By Wayne D. Guinn
|XRAMANIA- (CROSS-RAM-MA-NE-A) [GR., MADNESS] N. An extreme degree of enthusiasm
toward a special small block intake.
but true, once you've seen a Cross Ram intake manifold set up in an early Camaro engine
compartment, you can't help from being affected. Sometimes to the extent that it initiates
the bizarre (but extremely enjoyable) obsession defined above.
Most Camaro enthusiasts will readily admit
that one of their favorite topics of discussion at Camaro gatherings is the cross ram
induction system and how those cars equipped always get the draw...! And why not?
Aesthetically, the Cross Ram is awesome, the symmetry and proportion of the design is
strikingly beautiful, especially with the massive chrome topped air cleaner gleaming in
place. And function?... In terms of performance, the special induction system has
demonstrated itself to be capable of helping produce an amazing 458 HP from the 302 @ 7200
In fact... its sole purpose was to coax
huge amounts of horsepower from the relatively small 302 CID engine in the upper rpm range
without severely compromising low end power. And... if that wasn't a tall enough order, it
had to do so under the severe restriction of "keeping it in its pants" or.. more
accurately, under the hood due a safety rule imposed by the SCCA race sanctioning
As it turns out, not only was Chevrolet
able to accomplish all of the above, but they far exceeded all expectations and realized
tremendous success doing so. Today, we recognize the legendary Cross Ram Intake system as
being one of the largest contributing factors responsible for helping establish the Camaro
as the 1968-1969 TRANS-AM champion.
IT ALL BEGAN WITH THE NEED FOR...
Coming onto the
scene a few years behind the Mustang, the Camaro needed to play a hard and fast game of
Chevrolet would have to compete hard to
capture some sales from the "Pony Car" market that the Ford Mustang created. So,
when it came time to get competitive, Chevrolet got serious-both at the consumer level and
at the race track.
Chevrolets plan was simple... Their
competitive formula was based on the theory that a winning image sells- and it was through
racing that Chevrolet successfully promoted the Camaro and tapped into the fast growing
youth market. To create that image, Chevrolet put together the Camaro Z/28 package to
compete against their biggest rival, the Ford Mustang in the Sports Car Club of America's
(SCCA) Trans-Am road racing series, better known as "the battle ground".
Camaro's first year in competition was hard fought with a 302 single four barrel induction
system. This was the limit established by the SCCA sanctioning body in order to keep speed
at a reasonable rate for safety considerations. However, as the popularity of the series
increased, all caution went with the wind and for the 1968 Trans-Am racing season, the
SCCA changed the rules from "Not more than one four barrel" to "Not more
than two four barrels". This change came largely in response to unofficial factory
prodding with the intention of creating a "More exciting Trans-Am series".
The other manufacturers were ready and so
was Chevrolet. They released for use with the 302 cuin engine the "2X4
CARBURETOR CONVERSION UNIT" PN #3940077. The special "Cross Ram
Induction" system was made available over the counter through Chevrolet parts
departments as a "Heavy Duty Service" option beginning December 1967. Although
somewhat ambiguous, the term "heavy duty service", is Chevrolet's frequently
used catch all phrase meaning- "intended for racing".
The release of this special racing
equipment marked the begining of an incredible winning streak for the Camaro on the track
and for Chevrolet in terms of sales.
NOW, the nuts and bolts of it...
consists of a special two piece manifold, carburetors and related hardware needed for
installation. The inlet manifold assembly is a cast aluminum single plenum "Cross
Ram" type with provisions for two four barrel carburetors.
To the right is the Cross Ram System as it came through your local Chevrolet
By 1967, the Cross Ram concept was
by no means new, Chrysler engineers had developed the "RAM" idea in the early
'60's. Equal long tuned runners create a ram effect needed to "pack in" the fuel
mixture when using long duration cams on engines with high rpm capabilities.
above: Chrysler pioneered the use of the long- runner cross-ram intakes on
thier early '60s 300s as shown above...
Chevrolet Cross Ram design was intentionally kept low by placing the carburetors
"outboard" specifically for hood clearance. The vertical tunnel ram is more
efficient but it obstructs vision in road racing, for this reason the SCCA and NASCAR
rules required the manifold and carburetors to fit under a stock hood.
Cold air induction drawn through the cowl area was then
adapted to further enhance the 2X4 system. Once again this was nothing new, it was done
before in '63 on the Chevrolet Z11 stock cars in the NASCAR circuit. The high pressure
area at the base of the windshield is utilized to enhance the ram effect by allowing the
carburetors to draw fresh cool air which creates a denser fuel charge thereby increasing
volumetric efficiency and horsepower. Names synonymous with the system are: COLD AIR INDUCTION, FRESH AIR INDUCTION and COWL INDUCTION.
The offset carburetor configuration makes
it possible to use Holley carburetors with full metering bodies, allowing easy jet changes
for fine tuning, and center pivot float bowls needed to prevent flooding during hard
acceleration and cornering.
Although the manifold plenum chamber below
the carburetors is large, its size was kept minimal by its designers to retain good low
end response. The runner length was tuned specifically for use with the 302 cuin engine to
produce peak horsepower at 7,200 rpm using the optional "140" cam and tuned
headers. The full racing version of the 302/cross ram engine has an actual power curve
that is somewhat narrow and requires optimum use of gearing to maintain rpm in the power
range (4200-7200 RPM).
Chevrolet initially specified carburetor
#3941140, Holley list #3810S, which featured a 585 cfm rating, dual feed and sliding cam
actuated secondaries. Response problems led them to change to carburetor #3942595, Holley
list #4210A which featured a 600 cfm rating, dual feed, sliding cam actuated secondaries
with a different opening rate. Once again, response problems led to the third and final
change which cured the problem. Carburetor #3957859, Holley list #4295, rated at 600 cfm,
dual feed, sliding cam actuated secondaries and dual accelerator pumps did the trick.
Because of the relatively large plenum
area below the carburetors, accelerator pumps on primaries and secondaries are necessary
to adequately richen the mixture during acceleration to prevent an excessively lean fuel
mixture condition which causes bogging, power loss and ultimately burnt valves.
For optimum tuning the front carburetors
primaries face forward and the rear carburetors primaries face rearward. There are no
provisions for manifold heat or chokes on the carburetors, which is typical of a race only
Winters Foundry, who has done almost all
of Chevrolets aluminum casting, also produced the cross ram units. Development of the
Cross Ram was a collective effort on the part of many talented individuals inside and
outside of Chevrolet engineering including Smokey Yunick who performed test evaluations
and some "hands on" development.
After initial development was complete,
the prototype castings were test evaluated on the engine dynamometer. Studies revealed
"distribution fixes" were needed to prevent wet fuel travel. Chevrolet specified
a few additional changes over the initial prototype units which included the relocation of
the vacuum take off, from the bottom half of the manifold to the upper top plate, to
eliminate the possibility of wet fuel entering the vacuum lines and to gain a more direct
route for plumbing. Provisions for heater hoses were also added to facilitate the newer
water pump design and hose routing scheduled for use on the 1969 models.
The concept and development...
to engineer Bill Howell, the concept for the small block Cross Ram evolved from an
experimental tuned, long crossing runner, multi-carbureted manifold, that was developed in
latter part of '66 for the big block Mark IV engine. Coincidentally, the lobe profile for
the "140" off road cam which was developed for use in conjunction with the Cross
Ram, was also derived from the big block. Initial design work for the small block Cross
Ram began approximately mid year '67 with a target date for completion in December in
order to be homologated into the SCCA recognition forms in time for the start of the '68
Jerry Thompson, a key development
engineer, took the concept above and developed a test manifold using a Corvette fuel
injection bed plate. They epoxied in eight tubing runners to which they attached a plenum
box fabricated from sheet metal and fitted a top plate that would accommodate two 600 cfm
carburetors. Essentially, it was a vertical ram manifold. When it was set up on a
developmental test engine the dynamometer results showed an approximate 25.5 horsepower
increase over a base run using the same engine with the manifold/carburetor system used on
the '67 Trans-Am engines. This first crude adaption set the basic specifications for the
cross ram that was to follow.
The results of that test manifold were
exactly what they were after. The problem however was how to keep runner length long for
the ram effect, maintain the plenum box on which the carburetors sat, and somehow keep it
all under the hood. The solution along with the initial design specifications were drawn
up inside the drafting room by Design engineer Charley Turner. He was able to keep the
manifold height at a minimum and runner length long by laying the runners down, crossing
them horizontally side to side and then enclosing them by building the plenum box around
them. Top and side wall design as well as carburetor and linkage details were sorted out
by design engineer Chris Madson, followed by the laying up of cores and creation of plugs
at Winters Foundry. After the actual castings were produced at the foundry, they were sent
to Chevrolet Engineering and Smokey Yunicks garage for evaluation. They found out
immediately that the manifolds needed further refinement specifically in the area of fuel
distribution. Essentially, the problem was that the massive plenum area of the manifold
centrifuged the fuel mixture causing it to separate into it's two constituent components
leaning out the mixture markedly and causing wet fuel distribution on the sides and bottom
of the manifold. As a result of those findings several revisions were made, the most
important were being the fuel distribution fixes.
Project Engineer Jerry Thompson
unit would hardly run the mixture was so bad. We ran air flow tests and found it really
did pump the air. Using distribution fixes we tweaked it into a reasonable power band and
tried a collection of carbs. The 600 cfm's seemed adequate, larger carbs just ate up the
torque so we stuck with the 600's.
At this point, Rodger Penske and Mark
Donohue stopped by Engineering for a demonstration, Mark went nuts (9/16" wrench in
hand) to get the unit to use. We weren't quite done calibrating at the time so we held
them off. As soon as we had the calibration complete we had the foundry change the molds
and sent the prototype off to Traco Engineering (builders of Penske's engines). The
production people went on to dot the eyes and cross the tees as they do and the race cars
got the pieces they needed."
Initially there were four or five
"working prototype" manifolds that circulated back and forth between Smokey's
shop and Howell's group. One of the earlier prototype castings.
(10-27-67), was installed on the Chevrolet
built '68 Camaro Z/28 test vehicle that was used by Chevrolet Engineering for test
evaluation of the HD equipment. This car also doubled as a press car and was featured in
many magazine articles (SEE HOT ROD MAY '68) displaying the HD options for the '68 Z/28.
it would then be set up on the engine which was being prepared for Penske's '68 Trans-Am
clinching number 6 Camaro for the beginning of the rapidly approaching 1968 season.
Immediately following, specifications with revisions were drawn up for the production
version and manufacturing soon followed.
One of the most remarkable aspects
concerning the development of this special equipment is that the actual factory prototype
units were used on Penskes race cars long before they became production pieces, making it
truly a product of race bred technology.
note of interest concerning the early factory press release photographs depicting the
carburetors with milled air horns. According to Jim Travers of Traco Engineering, he
accompanied the Penske crew to the first race in which the Camaro used the Cross Ram
manifold system. They began experiencing some problems related to the induction system,
using a pair of Dutchmans (tin snips) Jim cut off the air horns and filed them smooth
relieving some of the problem, "Those first carburetors just wouldn't run right
As it turns out, the problem wasn't so
much the fault of the carburetor as it was with the first design air cleaner. Being the
element was only 1 1/2 inches tall, it bought the air cleaner lid so close to the
carburetor air horn, it restricted and upset flow to the primaries. The height restriction
forced air to flow horizontally and then make an acute turn down into the primary. This
sharp turning of air caused turbulence and upset proper fuel regulation within the
metering bodies. Cutting off the air horns smoothed out the flow and compensated for some
of the restriction.
The air horns function is to facilitate
the choke mechanism which on these carburetors is absent, not even the holes for the choke
butterfly shaft is drilled by Holley. Therefore, there is no need for the horn to remain
and it is beneficial to remove it for the sake of better flow. The problem is not as
critical with the second design air cleaner with the taller element, the additional height
of the element allows a more adequate, less restricted flow however, for competition this
modification serves a distinct advantage. Beyond the air horn the only other modification
Traco Engineering performed on the cross ram units was to stagger jet the carburetors for
a more even fuel mixture.
After the Traco prepared engines were
installed in the Penske cars they found that the production fuel lines developed for the
Cross ram unit began to present a problem during jet changes. The rigid steel lines and
fuel distribution block all tie together on the inside of the manifold between the two
carburetors, making access for maintenance/alterations difficult and time consuming.
According to Bill Howell they took the carburetor float bowls and switched them end to end
which puts the fuel lines on the outside of the manifold, effectively getting them out of
the way of the throttle linkage. Then by utilizing flexible aeroequip fuel lines, they
were afforded the convenience of quick and easy jet changes and float adjustments without
disconnecting the fuel lines.
The oil splash shield seen on the bottom
of portion the prototype Cross Ram unit was developed for testing. It was evaluated and
dropped never making production due to it's inability to demonstrate effectiveness during
testing. The small block does not have a problem of excessive amounts of oil thrown around
in that area as is common with the large block. The bottom of the manifold, which are
actually the intake runners, sits quite high and the small amount of oil splash that does
reach there does not significantly heat the manifold.
Chevrolet engineer in charge of cooling and lubrication development on the 302 engine
production vehicle the actual function of the manifold oil shield is to prevent oil from
coming in contact with the extreme heat of the manifold heat cross over. The inclusion of
which is necessary for good cold weather drivability. Any oil that would come in direct
contact with that super heated area would carburize, denaturing the oil and cause
"clinkers" to form and drop down into the engine case. Since there is no
manifold heat cross over used on the Cross Ram manifold, there is no need for a shield of
Stay Tuned for...
COMPLETING THE PACKAGE...
2X4 AIR CLEANERS and